It is often necessary to maintain samples such as electronic devices at very cold temperatures approaching absolute zero. For example, certain electronic devices must operate at low cryogenic temperatures of about 4 K to reduce electronic noise resulting from thermal fluctuations in circuits, and induce a superconducting state in electronic components. Cryogenic temperatures are also sometimes required to slow the rate of chemical reactions.
A conventional apparatus for lowering the temperature of samples to cryogenic levels employs cryostats which immerse the samples in cryogenic liquids such as nitrogen and helium to reach temperatures of -321.degree. F. and -452.degree. F., respectively. The use of such liquids, however, is complex, hazardous, expensive, and time-consuming. An example of a conventional immersive system is described in U.S. Pat. No. 4,848,093, which discloses a method of regulating the temperature of a cryogenic test chamber by controlling the flow of externally-supplied liquid helium.
Another conventional apparatus utilizing cryogenic liquids includes a dual chamber configuration with an inner chamber for housing a sample and an outer chamber enclosing the inner chamber. Liquid helium flows through the interior of a copper block exposed to the sample to cool the inner chamber.
A further conventional assembly employs cryostats using a closed cycle liquid helium generator as a cryopump to achieve non-immersive cooling with "cold finger" contact. Although cryopumps eliminate the need for cryogenic liquids, these systems introduce unacceptable strain into a sample as it is being cooled down from room temperature because the sample must physically contact the cooling element otherwise known as the "cold finger."
U.S. Pat. No. 4,872,321 addresses the problem of rigidly attaching devices to the cold finger of a cryopump in an evacuated sample chamber. As noted in U.S. Pat. No. 4,872,321, the stretching of tubing within a cryogenic precooler as gas is being cycled produces significant vibrational amplitudes at the cold finger which are transmitted directly to the devices/samples being cooled.
U.S. Pat. No. 4,872,321 teaches that the transmission of this vibrational amplitude to the devices can be prevented by mounting the devices from an independent support which is mechanically isolated from the portion of the precooler that is subject to the stretching vibration, namely the cold finger and adjacent portions of the precooler. Flexible coupling means, which connect the cold finger to the independent sample support through both a cryogenic gas supply and return line, specifically prevent transmission of stretching vibrations from the cold finger to the sample support and devices.
However, although mechanical coupling is reduced (but not eliminated) between the cold finger and the devices, vibrations from the piston or displacer in the cryopump are still transmitted to the device, and strain appears in the sample due to differences in the thermal expansion coefficients of the devices and cold finger.